1. FIELD OF THE INVENTION
The invention relates to optical filters utilizing atomic resonance transactions and more particularly to a filter which operates at one or more of the magnesium Fraunhofer lines in the green portion of the solar spectrum.
2. DESCRIPTION OF THE PRIOR ART
A critical need exists for the development of a reliable communications system between satellites in space and submarines operating in deep seawater during daylight hours. Atomic resonance filters are attractive due to the sharp filtering capability provided by absorption between atomic levels. This capability mitigates the undesirable effects caused by background solar radiation.
One type of communications system involves use of a satellite-based blue-green laser transmitting in the spectral range in which there is minimum sea water attenuation, namely between 450 to 550 nm, to a submarine-based narrowband atomic resonance filter with a large acceptance angle. Currently being studied is a particular atomic resonance filter which uses cesium or other alkali metals like rubidium or potassium as the atomic vapor in the atomic resonance filter.
There are several problems with the current set of atomic vapor absorption lines that have been proposed to date. The major problem is that the atomic resonance filter located on the submarine may be unable to detect the wavelength of the laser signal from the satellite due to the background noise created by the solar spectrum (400 nm to 700 nm) and by attenuation of the signal by seawater. For daytime transmission, the solar background determines the laser transmitter power necessary to close the communications link with a submerged submarine. Reduction of the effective solar background would yield significant benefits, including lower transmitter power as well as lower weight and volume and increased reliability for the space-based transmitter. Such reductions may be possible by transmitting in one of the very narrow Fraunhofer dips in the solar spectrum. However, the prior art in atomic resonance filters does not disclose the desirability of operating in those dips nor does it disclose an atomic vapor which is capable of operating at one of the precise wavelengths of the Fraunhofer dips.
A second problem with current atomic vapors is that even if the signal is strong enough to overcome the background noise and reaches the atomic filter, the output of the atomic filter may be at a wavelength that is inefficiently converted into electrical impulses. Conventional photomultiplier tubes operate best in the near ultraviolet (UV) region about 400 nm. At longer wavelengths, i.e. the near infrared, which are typically output from current atomic vapors, spectral response of these conventional tubes degrades. Although developmental photomultiplier tubes based on gallium arsenide technology offer promise of being able to detect wavelengths in the deep red (about 850 nm), such tubes are expensive, require cooling, are of limited surface area, and tend to degrade if accidentally exposed to high light levels.
A third problem is that the amount of information which can be transmitted is limited by the response time of the atomic vapor. Response time in an atomic resonance filter refers to the time it takes for the atomic vapor to complete the cycle of excitation and relaxation. A shorter response time of the atomic vapor increases the amount of information which can be transmitted. One factor that can slow reaction time is the presence of radiation trapping, which refers to the reabsorption of emitted light before it has a chance to leave the vapor cell. The current atomic vapors have relatively slow response times, usually in the range of about 10.sup.-6 seconds.
The atomic resonance filters disclosed in Marling, U.S. Pat. No. 4,292,526, have one or more of the above mentioned shortcomings. For example, Marling does not disclose any way other than transmitter power to overcome the solar background. In addition, the response time of cesium atomic vapor taught by Marling is relatively slow, about 2.times.10.sup.-6 seconds. This is due to severe radiation trapping of the emitted deep red light by the cesium vapor.
A principal object of the present invention is therefore to provide a filter which reduces the power demands of the satellite based transmitter through operation at a Fraunhofer dip in the solar spectrum.
It is another object of this invention to provide an output signal which is in that portion of the spectrum which is efficiently converted into electrical impulses by commercial sensing devices.
It is yet another object of this invention to maximize the amount of information that can be transmitted by a detected signal.